1. Field of the Invention
This invention generally relates to communication system link parameters and, more particularly, to a system and method for zeroing the gain level of a selected tap in a multi-tap transmitter, using commands from a communicating receiver.
2. Description of the Related Art
IEEE 802.3, Clause 72 defines a protocol, referred to hereafter as KR Training, to be used for a local device (LD) to adjust three different transmitter taps of a link partner (LP): the center tap, the post cursor tap (post-tap), and the precursor tap (pre-tap). The center tap settings are always a positive value, while the post cursor and precursor taps may be set to positive or negative values. When using KR Training to manipulate transmitter taps, three factors serve to limit the range of the transmitter taps. The first factor is the physical limit of the transmitter taps, referred to herein as the static limit. This limit is unique to each transmitter tap and is not dependent upon the value of the other transmitter taps. The other two factors are dynamic limits, in that these limits are dependent upon the current value of all the transmitter taps. One dynamic factor is the maximum swing voltage. This is the maximum voltage allowed by the transmitter. The output signal through the combination of transmitter taps must not exceed this voltage. The final factor limiting the transmitter taps is the minimum steady state voltage. This is the minimum voltage allowed by the transmitter. The center tap reduced by the absolute value of the post and pre cursor taps must not fall below this voltage.
For instance, reducing the precursor tap can cause the differential voltage resulting from combining all of the taps to reach the maximum limit. Once the combined maximum for all the taps is reached, the center tap cannot be increased, nor can the post cursor tap or pre-cursor tap be decreased. A tap change request can only change the value of a (requested) coefficient. The transmitter does not adjust the center tap, for example, in response to a pre/post cursor tap request.
FIG. 1 is a diagram depicting the maximum swing voltage and minimum steady state voltage limits (prior art). KR Training provides three possible transmitter responses to any tap request, they are: maximum, minimum, and updated. The maximum and minimum statuses indicate that the transmitter has been adjusted to reach a static or dynamic limit. Further, the maximum/minimum response gives no indication of whether the request was actually performed. That is, response may be the result of a tap change being performed, or if the transmitter was already in a limit condition, the response may be the result of no tap change.
During KR Training, it may be desirable to set the gain setting for a transmitter tap to zero. There are no status messages within the KR Training protocol to indicate when the zero setting of a transmitter tap has been reached. There is a tap request known as PRESET that may be sent by the LD to command the LP to set the center tap to its maximum value, and its pre and post-taps to zero. However, setting both the pre and post-taps of the LP to zero may not be feasible, or desired by the LD.
It would be advantageous if there was a methodology for individually setting the pre and post-taps of an LP transmitter to a zero gain setting.